Audio-frequency amplifier



June 20, 1950 R. L. UMBENHAUER AUDIO FREQUENCY AMPLIFIER Filed June 5, 1947 INVENTOR. ROBERT L. (/MfA/HA UER BY fi/H AMM ATTORN EYS.

Patented June 20, 1950 UNITED STATES PATENT OFFICE AUDIO-FREQUENCY AMPLIFIER Robert L. Umbenhauer, Barberton, Ohio Application June 5, 1947, Serial No. 752,599

amplifier capable of reproducing the full musical scale.

Conducive to a clearer understanding of this invention, it may be well to point out that in radio broadcasting we are interested in transmitting and reproducing as naturally as possible both speech and music. Sound waves are first changed into electriccurrents and then into radio waves at the transmitter. At the receiver the radio waves are'transformed into electric currents and finally backinto sound. 1 I

Th average fundamental frequency of the male voice is around 120 cycles per second, while that of the female Voice is about 240 cycles (an octave higher). However, harmonics exist in some speech sounds u to about 8000 cycles per second, and while-female speech has fewer overtones than the male, they extend up to 8000 cycles per second. The richest overtone-area .of -the male voice is between 3000 and 5000 cycles per second. Cutting off the frequencies above 6000 cycles per second eliminates the characterizing features of the unvoiced sounds such-as s, f, sh, th, z and the like. .These are absolutely necessary for the clear and distinct rendition of speech. Most of the energy of the voice occurs in the irequenciesbelow 1000 cycles per second, while most of theintelligibility occurs above that frequency. 7

The playing of a musical selection, by an orchestra, an organ ora piano involves the production of a large number of fundamental sound frequencies and. their accompanying higher harmonic frequencies. These musical tones are more or lesscomplex. Speech does notlinvolve as large a range of frequencies as. does music, so .thata system designed to satisfactorily.reproduce the entireuseful musical scale will. generally he satisfactory for speech also. r

The range. of fundamental. frequencies which must be transmitted in the reproduction of music fromv an entireorchestra will ordinarily range from about 40 up to 4000 cycles per second. The orchestra is composed-ofiour choirs, the strings, the wood-wind, the brass and the percussion. The lower andhigher strings .or keys on the harp and piano are seldom used However, satisfactory reproduction requires thatthe important harmonics. of the fundamental frequencies also. be reproduced, otherwise the reproduction will not possess the characteristics of the original sound, A certain amount of low frequency suppression is possible without serious 2 class. (01. 250-20 eiT ect, due to the fact thatthe ear has the power of supplying to our consciousness many of the fundamental; frequencies, provided the harmonies are reproduced. However, it is much better ifthesemissing fundamental frequenciesare reproduced, for the ear soon grows weary of performing thisiunction and the listener becomes mentally f-atigueda This tiring action is very markedwhen listening to.a conventional radio receiver which does not reproduce either the low or the high frequencies correctly, When the harmonics are not reproduced, the personal element in either the human voice or the musical instrument is lost. A violin. tune may sound like a whistle, while thehigh pitched tones of the piccolo may not be heard at all.

Conventional radio receivers, record players, and public address systems that do not reproduce the fullmusical scale attempt to overcome this deficiencybyhaving. atone control circuit which enables the listener to makea choice between reproduction with the bass over-accentuated and the high notes suppressed, or the bass reducedand thehigh. notes brought out, so that speech sounds such as 5, sh, ,z and the like are reproduced clearly and sharply. In either case, a part of the musical scale has to be lost in order. to obtain clarity of tone in the selectedrange.- 1

Heretofore, in order to reproduce the full musical scale it has been necessary to use two separate radio receivers tuned to the same musical program. One radio receiver being tuned to play all of the high notes, the other being tunedto, play all of the lownotes, By adjusting the volume ofjjeach radio receiver and playing both radio receivers simultaneously, the full musical scale can be obtained. This is true because one instrument is reproducing themiddle scale and highnotes while thaother instrument is reproducing themiddle scale and low notes.

.I-ioweyen'the use of two complete reproducing instruments is impractical and uneconomicah The,., primary object of this invention, there} fore, is to provide a single radio receiving instru mentthat will reproduce the entire musical scale from 40 to 2 0,000cycles per second. 7

fier.network, which amplifies these two separate bands and subsequently combines themin a single electro-acoustic transducer to reproduce the entire musical scale.

A further object is to provide a multistage audio frequency amplifier adaptable for use with record players, motion picture sound reproducers and public address systems that will amplify the entire musical scale without distortion.

Still another object is to provide an electronic discharge tube circuit that can be added to a conventional radio receiver or audio frequency amplifier for the purpose of converting it into an instrument that will amplify and reproduce the full musical scale from 40 cycles to 20,000 cycles per second.

These and other objects of the invention will become apparent from a reading of the following specification and claims, together with the accompanying drawing in which like parts are referred to and indicated by like reference characters and wherein:

Figure 1 is a schematic wiring diagram of a superheterodyne radio receiver made in accordance with this invention; and

Figure 2 is an enlarged view of the second detector network shown in the Figure 1.

Referring to'the drawing, there is shown in the Figure 1 a five-band superheterodyne receiver comprising a multi-selective carrier wave filter circuit of the conventional. type having the usual inductors, capacitors, resistors and switching arrangements, broadly indicated by the reference character I0.

The selected carrier wave is amplified at radio frequencies by the radio frequency amplifier tube I I. The output circuit of the amplifier H is coupled to a first demodulator and oscillator tube I2. The incoming signal is heterodyned to a lower intermediate frequency signal which is impressed upon the input circuit of the intermediate frequency amplifier 13. The output of the intermediate frequency amplifier I3 is fed to the second demodulator network broadly indicated by reference character M. This network contains two electron discharge tubes 20 and 2| which serve as demodulators and also as first audio frequency amplifiers. The details of this circuit are shown in the Figure 2 and will be disclosed in detail hereinafter. Reference character 15 indicates asecond stage resistancecoupled audio frequency amplifier whose output is fed to the phase inverter Hi. The third stage of audio frequency amplification comprises two resistance coupled power amplifier tubes l1 and 18 connected in a push-pull circuitwhose output is fed to the electroacoustic transducer or loud speaker 10 thru the output transformer 1|.

-Reference character l9,.indicates a full wave power rectifier tube and its accompanying filter and voltage divider system One side of the power supply circuit is grounded to the chassis l of the instrument which serves as a common lead for the various circuits. f

The second demodulator and first amplifier network shown in the Figure 2 comprises two similar .electron dischargetubes 20 and 21, each having a heater 22, a cathode 23, a control grid 24, a rectifier plate 25 and a plate 26. Reference character R! indicates a 10 megohm resistor; R2 indicates a 100,000.0hm resistor; R3 indicates a 2 megohm volume controlgRA indicates a 470,000 ohm resistor, and C indicates a .02 microfarad fixed capacitor. 3

The two tubes are connected as shown with their common input 40 connected to the rectifier plates 25 of the two tubes 2t and 2!. The plate circuits 50 and 5! of the tubes 20, and 2! respectively, each contain a 100,000 ohm resistor R2. The two plate circuits are connected thru a common output circuit 60 to the input of the second audio amplifier tube [5.

The plate circuit 50 has a capacitor 52 of .02 microfarad capacitance connected between it and the ground I. The plate circuit 5! has a capacitor 53 of .00025 micromi-crofarad capacitance connected between it and the ground I.

The intermediate frequency currents leaving the intermediate frequency amplifier [3 enter the second demodulator network 14 thru the input circuit 40 where they are rectified and amplified by the tubes 20 and 2|.

Audio frequency currents of from 13,000 to 20,000 cycles per second in the plate circuit 50 of the tube 20 find less impedance to their progress thru the capacitor 52 than thru the 100,000 ohm resistor R2, therefore currents between these frequencies pass thru the capacitor 52 to the ground 1. However, audio frequency currents of from 40 to 13,000 cycles per second find more impedance to their progress thru the capacitor 52 than thru the 100,000 ohm resistor R2. Therefore, these low frequency audio frequency currents will pass thru the plate circuit 50, the output circuit 50 and into the following audio frequency amplifier circuits.

In the plate circuit 5! of the tube 2! audio frequency currents of from 40 to 20,000 cycles per second find more impedance to their progress thru the capacitor 53 than thru the 100,000 ohm resistor R2. Therefore, all frequencies from 40 to 20,000 cycles per second will flow thru the plate circuit 5! and into the common output circuit 60. Leaving the output circuit 60 they enter the following audio frequency amplifier circuits. Unrectified intermediate frequency currents that might cause distortion in the later reproduced audible sound pass thru the capacitor 53 to the ground 1.

Thus it will be readily seen that by using two demodulators at the beginning of the audio frequency amplifier and controlling one demodulator for low notes and controlling the other demodulator for high notes, the single audio frequency amplifier becomes two audio frequency amplifiers and produces both the high and the low notes or the complete musical scale. Inasmuch as the audio frequency currents that energize the loud speaker !0 have a range of frequencies from 40 to 20,000 cycles per second, the loud speaker will produce sound that is brilliant and natural since the entire musical scale is being reproduced. The individual volume controls R3 permit the volume of the notes in both the high and low range of the musical scale to be adjusted to suit the listener. The tone control filters or capacitors 52 and 53 are fixed and there are no manually operated tone controls.

The values of the capacitors 52 and 53 given are intended for use with the superheterodyne circuit illustrated. When used with other circuits, the value of capacitor 52 may vary between 0.5 microfarad and 0.005 microfarad, while that of capacitor 53 may vary between 0.005 microfarad and 0.00025 micromicrofarad.

The elements of the two tubes in my circuit can be enclosed in a single envelope if desired for the izke of compactness and economy of manufac- This audio frequency band separator circuit may be used with any existing radio by the addition of a second demodulator tube of the same electrical characteristics as the tube already present in the set. The two demodulators are then connected as shown in the Figure 2. The two audio bands of low and high notes are amplified separately thru the existing single audio amplifier to reproduce the entire musical scale.

My invention may be adapted for use with audio amplifiers of the type used in record players, moving picture sound reproducers, public address systems and ore-amplifiers for broadcast transmitters by duplicating the first tube of the amplifier and setting up the circuit shown in the Figure 2.

It will now be clear that there is provided a device which accomplishes the objectives heretofore set forth. While the invention has been disclosed in its preferred form, it should be understood that the specific embodiment thereof as described and illustrated herein is not to be considered in a limited sense, as there may be other forms or modifications of the invention which should also be construed to come within the scope of the appended claims.

I claim:

1. In a radio receiver for receiving a source of modulated radio frequency signal, means for separating, detecting, amplifying and filtering said signal in separate paths, comprising a pair of separate rectifier means connected to said signal source, each separately demodulating said signal, separate amplifier means connected to each rectifier output for amplifying the audio signal therefrom, a filter for passing frequencies between 40 and 13,000 cycles per second connected in the output of one of said amplifier means and a filter for passing frequencies between 40 and 20,000 cycles per second connected in the output of said other amplifier means, and means connecting said filter outputs together for re-combining said signal.

2. In a. radio receiver for receiving a source of modulated radio frequency signal, means for separating, detecting, amplifying and filtering said signal in separate paths, comprising, a pair of separate rectifier means connected to said signal source, each separately demodulating said signal, separate amplifier means connected to each rectifier output for amplifying the audio signal therefrom, a filter for passing frequencies between and 13,000 cycles per second connected in the output of one of said amplifier means, comprising a capacitance shunted across said amplifier output and a resistance in series with said output, and a filter for passing frequencies between 40 and 20,000 cycles per second connected in the output of said other amplifier means, comprising a small capacitance shunted across said other amplifier output and a resistance in series with said output, and means for connecting saidfilter output together for recombining said signal.

ROBERT L. UIWBENHAUER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,023,511 Lewis Jan. 21, 1936 2,173,925 Tuxen Sept. 26, 1939 2,235,859 Wilson Mar. 25, 1941 2,366,565 Shea Jan. 2, 1945 FOREIGN PATENTS Number Country Date 680,755 Germany Sept. 7, 1939 152,402 Austria Feb. 10, 1938 

